Rare earth metal-based permanent magnets such as R—Fe—B based permanent magnets, which are represented by Nd—Fe—B based permanent magnet, possess high magnetic properties and are now utilized in various fields. In particular, bonded magnets using magnetic powder and resin binders as the principal components are excellent in dimensional precision and can be easily tailored into desired shapes or into monolithic molding. Therefore, a variety of shaped bonded magnets such as ring-shaped magnet are used widely in electronic appliances and the like.
However, rare earthmetal-based permanent magnets contain metallic species (particularly R) prone to oxidative corrosion under the atmosphere. Thus, in case they are used without applying surface treatment, corrosion proceeds from the surface due to the effect of acids, alkalis, water, and the like that are slightly present in air, and rust generates as a result. This causes deterioration or fluctuation in magnetic properties. Moreover, in case rust generates on magnets assembled in devices such as magnetic circuits, it is feared that rust is scattered to contaminate peripheral components. Accordingly, in case of using rare earthmetal-based permanent magnets, it is required to impart corrosion resistance to the magnet.
Various methods are proposed to present for imparting corrosion resistance to an article to be plated, e.g., a rare earth metal-based permanent magnet, and a method of forming copper plating film on a surface of an article to be plated by electroless plating method is one of such methods.
In the copper plating method using electroless plating method, generally employed is the chemical reduction plating method, which comprises adding a reducing agent in a copper plating solution containing copper sulfate and ethylenediaminetetraacetic acid as the principal components, and then, by using the reducing agent, selectively depositing copper ions in the copper plating solution on a catalyst-activated surface of the article to be plated. According to this method, the copper deposition reaction continues so long as the copper deposit remains catalytically active with respect to the oxidation reaction of the reducing agent. Thus, a copper plating film of desired film thickness can be formed on the surface of the article to be plated in accordance with the time duration of immersing the article in the copper plating solution. In this method, formaldehyde is generally used as the reducing agent in an alkaline bath condition.
As a method for forming a copper plating film on the surface of an article to be plated, the copper plating method employing chemical reduction plating method as described above is of high practical value. However, in case of the reductive deposition reaction using formaldehyde, gaseous hydrogen generates according to the reaction mechanism. The bubbles of gaseous hydrogen negatively influence the adhesion of copper plating film onto the articles to be plated, particularly those having concave portions such as voids on the surface, such as bonded magnets. Moreover, in case of the articles to be plated that undergo hydrogen embrittlement, such as rare earth metal-based permanent magnets, gaseous hydrogen casts unfavorable effects to the articles themselves.
Accordingly, an object of the present invention is to provide a copper plating solution and a method for copper plating that stably allow formation of a uniform copper plating film excellent in adhesion on the surface of an article to be plated, such as rare earth metal-based permanent magnet.